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Impact of haemostatic mechanisms on pathophysiology of preeclampsia

Chris Gardiner1, Manu Vatish2

1Haemostasis Research Unit, Department of Haematology, University College London, UK. 2Nuffield Department of Obstetrics and Gynaecology, University of Oxford, Oxford, UK. Corresponding author. Dr Chris Gardiner PhD, Haemostasis Research Unit, Department of Haematology, University College London1st Floor, 51 Chenies Mews,London WC1E 6HX. UK Phone: +44 (0)20 7679 6417 Fax: +44 (0)20 7679 6424

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Placental formation Humans have haemochorial placentation in which the placenta is in direct contact with circulating

maternal blood. The placenta comprises fetal vasculature, stroma, immune cells and cytotrophoblast

which are extravillous and villous. During implantation, the maternal decidual arteries are invaded

by placental extravillous cytotrophoblasst resulting in the subsequent remodelling of the decidual

spiral arteries. During this process, placental extravillous cytotrophoblasts replace maternal

endothelial cells and disorganise the vascular smooth muscle cells (VSMC). This leads to a high flow

and low resistance placental circulation enabling effective exchange of nutrients across the chorionic

villi. The chorionic villi are covered in syncytiotrophoblasts, which are formed from a villous

cytotrophoblast layer, in direct contact with the maternal blood, and thus trophoblasts, rather than

the endothelium, form the exchange surface in the intervillous space and act as the gatekeeper

between maternal blood and embryonic tissues.

During implantation, tissue factor (TF) and plasminogen activator inhibitor-1 (PAI-1) play a key role in

creating a haemostatic envelope around the invading blastocyst thus preventing local decidual

haemorrhage [1-3]. Unlike other vascular beds, trophoblasts constitutively express TF. In normal

pregnancies TF procoagulant activity is held in check as the syncytiotrophoblast adapts a

pseudoendothelial phenotype through the expression of integrin αv/β3, PECAM-1 and E-cadherin

[4], endothelial protein C receptor (EPCR), thrombomodulin, annexin-V and the tissue factor

pathway inhibitors (TFPI-1 and TFPI-2). This prevents TF induced thrombin generation in healthy

pregnancies [5]. TF is essential for fetal and placental development and the prevention of placental

abruption in later pregnancy, as demonstrated by fetal lethality in TF knockout mice and supported

by the absence of reported deficiency states in humans [2].

The protease activated receptors PAR-1, PAR-2 and PAR-3 are also expressed on extravillous

trophoblasts (EVT) and syncytiotrophoblasts [6, 7] and mediate cytotrophoblast invasion via

thrombin (PAR-1 and PAR-3) and TF/FVIIa or TF/FXa (PAR-2) activation, whilst PAR-1 simultaneously

induces differentiation through nuclear recruitment of β -catenin and induction of Wnt-dependent

T-cell factor 4 [6, 8, 9].

Pre-eclampsia

Pre-eclampsia (PE) is a disorder of new onset hypertension and proteinuria, thrombocytopenia,

renal insufficiency or impaired liver function in the second half of pregnancy [10]. The origins of PE

lie in the placenta, and delivery remains the only effective treatment. PE is a complex syndrome

with several overlapping subtypes, e.g. early onset or late onset, with or without fetal

growth restriction (FGR) [11]. PE has been described as a two stage disease [12] with a first

(pre-clinical) stage comprising deficient remodelling of the uteroplacental spiral arteries (8–

18 weeks), dysfunctional perfusion and placental oxidative stress leading to the clinical

stage (after 20 weeks) characterised by maternal systemic inflammation and vascular

dysfunction. While this is undoubtedly true for early onset disease (<34 weeks gestation),

many cases of late onset PE have no antecedent placental pathology [13] and appear as two

distinct phenotypes as indicated by the placental histology. In general, placentae from early

onset PE are smaller than aged matched controls [14],with more infarction and are less well

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developed, with reduced terminal villi volume and surface area [15, 16]. In contrast, late

onset PE placentas are frequently larger with similar morphology to normal term placentas

and some show increased arteriopathy [16].

The diverse nature of PE is also reflected by the maternal haemodynamic state. Early onset

PE and, to a lesser extent, normotensive FGR are preceded by increased total vascular

resistance and low cardiac output, where as those women destined to develop late onset PE

are characterised by low total vascular resistance and high cardiac output [17]. This has led

to the concept of “placental PE” in which an abnormal placenta interacts with normal

maternal vasculature leading to early onset disease, and “maternal PE” in which a normal

placenta interacts with abnormal vasculature to produce late onset disease [18]. Although

the aetiology of the different subtypes is clearly distinct, the maternal syndrome of

hypertension, proteinuria and oedema due as a result of vascular inflammation and

endothelial dysfunction represent a common pathway. This is driven by increased placental

release of the soluble isoforms of the vascular endothelial growth factor receptor 1

(sVEGFR1 or sFlt-1) and endoglin (soluble TGFβ-1 receptor) with reduced placental growth

factor (PlGF) [18].

Perhaps unsurprisingly, given the heterogeneous nature of the condition, many pathogenic

mechanisms have been proposed and these have been recently reviewed [19].This review

will concentrate on haemostatic mechanisms responsible for placental dysfunction.

As previously discussed, tissue factor is essential for placental development. PAR signalling is

important in trophoblast differentiation and a failure of trophoblasts to adopt an

endothelial phenotype is associated with PE [4]. However, increased TF expression

[2](Girardi, 2010) and/or exposure of trophoblast TF to FVIIa during decidual haemorrhage

causes inappropriate activation of the coagulation cascade and this has been proposed as

the primary cause of impaired trophoblast invasion of the decidua (Lockwood et al., 2011).

Increased expression of TF in endothelium of the basal decidua is associated with bilateral

notching of the uterine artery and severe PE with FGR [20]. Aberrant syncytiotrophoblast TF

expression has been reported in PE [21, 22] with a corresponding reduction in TFPI1 [21]

and TFPI2 [23, 24]. We have also demonstrated increased tissue factor activity on

syncytiotrophoblast extracellular vesicles (STEV) released into the maternal circulation from

the placenta in PE [25].

Over expression of villous trophoblast PAR-1 is also a feature of early onset preeclampsia

[26]. Excessive thrombin activation of PAR-1 enhances sflt-1 expression and secretion by

trophoblasts through the PAR-1 /NADPH oxidase /ROS signalling pathway [27, 28](Zhao et

al., 2012, Huang et al., 2015). This is evinced by increased maternal markers of in vivo

thrombin generation and fibrin turnover [29-32]. This represents a clear link between

increased placental TF and the resultant thrombin activation of PAR-1 [33] leading to excess

sflt-1 secretion and maternal endothelial dysfunction. Perhaps of more importance to the

foetus is the localised effect of thrombin generation in the placenta. Perivillous fibrin

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deposition and placental infarction are strongly associated with early onset PE and severe

disease [34] [35-37].

Infarction may lead to localised hypoxia and the expression of hypoxia-inducible factor 1-

alpha (HIF-1α). Several studies have shown that women with PE have persistently elevated

placental HIF-1α which promotes enhanced transcription of genes encoding sFlt-1, endoglin

and endothelin-1, all of which are known to contribute to preeclampsia [38-40]. Moreover,

expression of HIF-1α is upregulated not only by hypoxia, but also by inflammatory stimuli

(for example, thrombin, vasoactive peptides, cytokines, such as TNFα, and reactive oxygen

species [ROS]), especially those mediated by NF-κB, as the promoter of HIF-1α contains an

NF-κB binding site [19].

Changes in systemic maternal haemostasis Normal pregnancy is associated with a physiological increase in many procoagulant factors and

inhibitors of fibrinolysis. Increases in coagulation factors VII, VIII, X, XII and XIII, fibrinogen and von

Willebrand factor are all commonly observed and are maximal around term. Free protein S

decreases throughout pregnancy and this is reflected by increasing resistance to activated protein C

(APC). Fibrinolytic activity is reduced during pregnancy, as a result of increased levels of plasminogen

activator inhibitor-1 (PAI-1) from endothelial cells and plasminogen activator inhibitor-2 (PAI-2) from

the placenta [41]. These changes are necessary in order to meet the haemostatic challenge of

parturition but contribute to an increased risk of venous thromboembolism during pregnancy and

the puerperium. A benign gestational thrombocytopenia is not infrequently observed, particularly in

the third trimester but this is not associated with excessive platelet activation and is most probably

due to increased plasma volume. As might be expected, markers of in vivo thrombin generation

(thrombin-antithrombin complexes [TAT] and prothrombin fragment 1.2 [PF1.2]) are slightly

increased as is the endogenous thrombin potential (ETP) assay [42].

The association between coagulation activation and preeclampsia has been recognised since the

early 1950s. The shift in the haemostatic balance towards a procoagulant/hyperfibrinolytic picture

observed in normal pregnancy is exaggerated in preeclampsia. Many consider the haemostatic

changes observed in mild preeclampsia as an augmentation of the normal maternal physiological

response. This is in contrast to severe preeclampsia in which the imbalance of the haemostatic

system is pathological, reflecting the systemic inflammation and endothelial dysfunction

characteristic of the disease. In practice, a spectrum of haemostatic changes are observed; from the

subtle variations seen in mild preeclampsia to the unregulated disseminated intravascular

coagulation (DIC) observed in the HELLP (haemolysis, elevated liver enzymes and low platelets)

syndrome. Increased thrombomodulin activity, tissue factor activity and procoagulant phospholipids

are observed [43, 44].

The role of platelets The association of excessive platelet activation and consumption in PE has been recognised for many

years [45, 46]. There is now ample evidence to implicate platelet activation in the pathogenesis of

PE. Platelet activation may occur several weeks prior to the onset of symptomatic PE [47, 48];

Increased mean platelet volume in late first trimester of pregnancy has been reported to predict

intrauterine growth restriction and PE [49]; Platelet-derived soluble factors are reported to regulate

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human extravillous trophoblast differentiation and invasion into maternal spiral arteries [50]; Low

dose aspirin (LDA) initiated before the 16th week of gestation significantly reduces the incidence and

severity of preeclampsia, FGR and preterm birth, whereas LDA initiated after the 16th week has little

effect on pregnancy outcome [51]; Increased platelet-leucocyte aggregates are observed in PE

compared to normotensive pregnancies [44, 52] and these have been identified as a significant

source of extraplacental sflt-1 in women with PE during pregnancy and for several months post-

partum [53, 54].

We have demonstrated that platelets take up extracellular vesicles derived from the

syncytiotrophoblast (STEVs) in vitro [55, 56] and that placenta-specific proteins and mRNA are

detectable in platelets isolated from the peripheral blood of pregnant women. Furthermore,

incubation of platelets with STEV leads to platelet degranulation in vitro (unpublished data). This is

consistent with previous reports of reduced aggregation response to weak agonists in platelets from

PE women due to “exhausted” platelets [57-60]. It is possible that systemic release of vasoactive

substances by platelets during PE may contribute to vascular dysfunction. Most recently, it has been

shown that endothelial EVs cause the accumulation of activated platelets within the placental

vascular bed leading to inflammasome activation in trophoblasts in a mouse model of PE [61].

The role of neutrophils Normal pregnancy is characterized by the presence of innate immune cells at the feto-maternal

interface from the first trimester. It has been proposed that TLR stimulated trophoblasts recruit

neutrophils via the release of IL-8 [62]. IL-8 and STEV released from placental explants induce the

formation of neutrophil extracellular traps (NETs) in vitro [63]. Furthermore NETs have been

demonstrated in the intervillous space of placentae from pre-eclamptic women and it has been

proposed that NET formation may be responsible for the increased cell-free DNA observed in the

plasma of women with PE [64]. As NETs are known to promote thrombosis in a P-selectin-dependent

manner [65, 66] it is possible that NET formation may contribute to placental infarction.

In a rat model of PE, neutrophil depletion was shown to attenuate placental ischaemia-associated

hypertension, suggesting a significant role for neutrophils in the pathogenesis of PE [67]. It has been

reported that activated endothelial cells induce NET formation which in turn causes further

endothelial damage [68]. This, and the association of NETs with thrombotic microangiopathies [69],

suggest a role for systemic NET involvement in fulminating PE and HELLP syndrome.

Anti-phospholipid antibodies and PE Maternal antiphospholipid antibodies (aPL) are associated with recurrent miscarriage, intrauterine

death, preeclampsia, intrauterine growth restriction and premature birth [70]. For many years, it has

been assumed that the role of aPL in the pathogenesis obstetric antiphospholipid syndrome (APS) is

primarily thrombotic. However, recent findings question the validity of this assumption. Combined

LDA and heparin are effective in improving pregnancy outcome in women with APS and a history of

early pregnancy loss [71-73] but women with APS and prior fetal loss remain at high risk of placenta-

mediated complications despite treatment with LDA and LMWH [74].

Heparin appears to exert its protective effect through the inhibition of complement activation [75].

aPL cause a complement-independent inflammatory response by signalling through TLR-4 on EVT

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[76] whereas pregnancies in mice infused with non-complement-fixing β2GP1 single chain antibody

were unaffected [77]. Furthermore, inhibition of C3 convertase prevented fetal loss and growth

retardation in mice injected with human aPL [78]. A recent clinical trial reported pravastatin

improved pregnancy outcome in women with APS who were refractory to LMW/LDA and previously

developed PE [79]

Conclusions

It is clear that haemostatic mechanisms play an important role in the pathogenesis of PE. It is hoped

that our increased knowledge in this field will identify new therapeutic strategies for the treatment

and prevention of this dangerous condition.

Conflict of interest statement

The University College London has received an unrestricted educational grant from Sysmex Inc. in

the last 12 months. MV has received research support from Roche Diagnostics.

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Figure legend

Figure 1. Haemostatic mechanisms in placental insufficiency. Syncytiotrophoblast stress induces TF

expression and activation of complement factors C3 and C5. This causes neutrophil recruitment,

activation and oxidative burst. TF positive STBEV bind platelets triggering the release of ADP and

thrombin generation and. Thrombin activates PAR1 receptors on the Syncytiotrophoblast, platelets

and neutrophils causing further cellular activation. The resulting syncytiotrophoblast inflammation

promotes increased shedding of TF STBEV and release of sFlt-1.

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